CN107206494B

CN107206494B - Additive manufacturing apparatus and associated methods utilizing a particular scanning strategy

Info

Publication number: CN107206494B
Application number: CN201580074172.6A
Authority: CN
Inventors: 迈克尔·约瑟夫·麦克利兰; 凯里·布朗; R·G·阿斯瓦塔纳拉扬斯瓦米
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 2014-11-21
Filing date: 2015-11-17
Publication date: 2021-03-26
Anticipated expiration: 2035-11-17
Also published as: EP3689507A1; JP2018502216A; US11267052B2; US10500641B2; PL3221075T3; US20200070247A1; CN107206494A; EP3221075A2; US20180290241A1; WO2016079496A2; GB201420717D0; WO2016079496A3; EP3221075B1; ES2807596T3

Abstract

The invention relates to an additive manufacturing apparatus comprising: a build chamber (101); a build platform (102) which is lowerable in the build chamber (101) such that a layer of flowable material can be continuously formed across the build platform (102); a laser (105) for generating a laser beam (118); a scanning unit (106) for directing the laser beam (118) onto each layer to selectively cure the material; and a processor (131) for controlling the scanning unit (105). The processor (131) is arranged to control the scanning unit (105) to direct the laser beam (118) to solidify a selected region of material by advancing the laser beam (118) a plurality of times along a scan path (200). At each scan along the scan path (200), the laser beam (118) solidifies spaced-apart sections of the scan path (200), and each subsequent scan solidifies sections between the sections solidified by the previous scan. In another embodiment, the additive manufacturing apparatus comprises: a laser source for generating a plurality of laser beams; the processor (131) is arranged to control the scanning unit (106) to direct the laser beams to solidify selected areas of material by successively advancing a plurality of the laser beams along a scan path, wherein as each of the laser beams scans along the scan path, the laser beam solidifies spaced apart sections of the scan path, and scanning of one of the laser beams along the scan path solidifies sections located between sections of the scan path solidified by another of the laser beams. The processor (131) may be arranged to control the scanning unit to direct the laser beam to solidify selected areas of material by solidifying sub-millimeter sized sections of the area non-continuously and sequentially so that successively solidified sections are spaced apart.

Description

Additive manufacturing apparatus and associated methods utilizing a particular scanning strategy

Technical Field

The present invention relates to an additive manufacturing apparatus and method of solidifying layers of material in a layer-by-layer manner to form an object. The invention has particular, but not exclusive, application to selective laser solidification apparatus, for example, Selective Laser Melting (SLM) and Selective Laser Sintering (SLS) apparatus.

Background

Selective Laser Melting (SLM) and Selective Laser Sintering (SLS) devices use a high energy beam (e.g., a laser beam) to produce an object via layer-by-layer solidification of a material (e.g., a metal powder material). A powder layer is formed across a powder bed in a build chamber by depositing a powder mass adjacent to the powder bed and spreading the powder mass over the powder bed (from one side of the powder bed to the other) with a scraper to form a layer. The laser beam is then scanned across an area of the powder layer corresponding to a cross-section of the built object. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is reduced by the thickness of the newly solidified layer and another powder layer is spread over the surface and solidified as needed. An example of such a device is disclosed in US 6042774.

Typically, the laser beam is scanned across the powder along a scan path. The arrangement of the scan paths will be defined by the scan strategy. US5155324 describes a scanning strategy which comprises scanning the contour (boundary) of a partial cross-section followed by scanning the interior (core) of the partial cross-section. Scanning the boundary of the portion may improve the resolution, clarity and smoothness of the surface of the portion.

It is known to use a continuous-type laser mode of operation, in which the laser is kept on while the mirror is moved to direct the laser spot along a scan path; or using a pulsed laser mode of operation in which the laser is pulsed on and off as the mirror directs the laser spot to different positions along the scan path.

The strategy used to scan the portions can affect the heat load generated during construction and the accuracy of the resulting cured lines of material.

Excessive unconstrained thermal stresses generated during construction cause the constructed part to deform and/or curl. As the solidified material cools, the temperature gradient of the cooling solidified material may cause deformation and/or curling of the portion. US5155324 and US2008/0241392 a1 describe scanning an area in a plurality of parallel scan paths (raster scan). The direction of the scan path is rotated between the layers to even out the tension created during construction. US2008/0241392 a1 extends this concept to the scanning of a series of parallel swaths, where each swath consists of multiple parallel scan paths running perpendicular to the longitudinal direction of the swath. The direction of the strip was rotated 67 degrees between the layers.

US2005/0142024 discloses a scanning strategy for reducing the heat load comprising successively radiating individual areas of a layer, which areas are at a distance from each other that is larger than or at least equal to the average diameter of the individual areas. Each individual region is irradiated in a series of parallel scan paths.

The molten pool created by the laser depends on the properties of the material and the state (powder or solidified) and temperature of the material surrounding the volume being melted. The scanning strategy used may affect the state and temperature of adjacent materials. For example, scanning the laser spot along the scan path in a continuous mode forms a larger molten pool that is dragged immediately behind the laser spot, resulting in a larger, less detailed solidification line. For some materials (e.g., tool steel and aircraft grade superalloys), it may be difficult to drag the molten pool across the layers in a continuous mode of laser operation. These problems can be alleviated by using the laser beam in a pulsed mode of operation. In particular, setting the time between pulses long enough to allow a previously formed melt pool to cool before forming an adjacent melt pool can produce more accurate solidification lines, which can be particularly beneficial for boundary scanning. Slowing the scan to this extent, however, can significantly increase the time to scan that area/path and thus significantly increase the build time.

Disclosure of Invention

According to a first aspect of the invention, there is provided an additive manufacturing apparatus comprising: a build chamber; a build platform that is lowerable in the build chamber such that a layer of flowable material can be continuously formed across the build platform; a laser for generating a laser beam; a scanning unit for directing the laser beam onto each layer to selectively cure the material; and a processor for controlling the scanning unit.

The processor may be arranged to control the scanning unit to direct the laser beam to solidify selected areas of material by advancing the laser beam along a scan path a plurality of times, wherein on each scan along the scan path the laser beam solidifies spaced apart sections of the scan path, each subsequent scan solidifying sections located between the sections solidified by the previous scan.

The additive manufacturing apparatus may comprise: a laser source for generating a plurality of laser beams; a scanning unit arranged to direct the laser beam onto each layer to selectively cure the material; and a processor arranged to control the scanning unit to direct the laser beam to solidify selected areas of material by successively advancing multiple ones of the laser beams along a scan path, wherein on a scan of each of the laser beams along the scan path, the laser beam solidifies spaced apart sections of the scan path, and a scan of one of the laser beams along the scan path solidifies sections between sections of the scan path solidified by another one of the laser beams.

The scan path may be a boundary scan path around the boundary of the selected area. Performing such a scan may increase build time as compared to forming a continuous curing line along the scan path. Therefore, for the core of the selected area, a more efficient scanning strategy is preferably used. However, highly precise melting is required at the boundaries of the region, and the scanning method according to the invention may achieve an improved accuracy of the scanning along the boundaries. However, in some cases it may be desirable to use such a scanning strategy for the core of the area to be cured. For example, for materials that are difficult to process by scanning strategies that solidify the material with large continuous lines (hatched lines), such as tool steels and aircraft grade superalloys, this scanning strategy can also be used in the core of the area to be solidified.

A first scan of the laser beam along the scan path may be in a first direction and a subsequent scan (e.g. a second scan) of the laser beam or another laser beam along the scan path may be in a second, opposite direction. For example, for a boundary scan, the first scan may be in a clockwise/counterclockwise direction around the boundary, and the second scan may be in the other of the counterclockwise/clockwise directions.

The processor may be arranged to control the scanning unit to direct the laser to polarise the selected region of material by curing sub-millimetre sized sections of the region non-continuously and sequentially so that the continuously cured sections are spaced apart.

In this way, while a previously irradiated section of the laser beam is cured, another section spaced apart from the previously irradiated section is irradiated with the laser beam. Thus, curing delays for selected areas are reduced, while avoiding inaccuracies and thermal stresses created by continuously scanning large sections, as compared to waiting for curing of a previous section before irradiating an adjacent section.

By virtue of these small segments, a more isotropic cured segment can be formed than a longer segment. It is to be understood that "sub-millimeter sized sections" means that all dimensions of the sections are less than 1 mm.

Each segment may be formed (e.g. in the form of a line) by irradiating a single spot with a (static) laser beam or moving a laser beam across the layer. The segments may be sized only when the material is cured. In one embodiment, the segments may be sized such that irradiating the segments with the laser beam creates a molten pool that extends across the entire segment.

The processor may be arranged to control the scanning unit to direct the laser beam to solidify selected areas of the material by irradiating sections of the area with the laser beam, so as to allow each irradiated section to be solidified before adjacent sections are irradiated by the laser beam or another laser beam.

Each section of a selected area of one layer may be arranged to (only) partially overlap with a section of a corresponding selected area of a preceding layer. Each section may be substantially circular dots, the dots of each layer being arranged in a regular pattern, with the pattern of one layer being offset relative to the corresponding pattern of the previous layer. The dots may be arranged in a triangular pattern. The dots of the pattern may be cured in order such that adjacent dots are not cured in sequence.

The dots of the pattern are irradiated with the laser beam or beams advancing across the pattern in a direction that is different from (in the same way as) the direction in which the dots are irradiated with the pattern in the corresponding selected area of the preceding layer. For a triangular pattern, the direction of radiation progression for the dots may vary by 60 degrees or 120 degrees between each layer.

According to a second aspect of the present invention there is provided a method of scanning layers of material in a layer-by-layer additive manufacturing process in which successive layers of flowable material across a build platform are formed and a laser beam is scanned across selected regions of each layer to solidify the material in selected regions.

The method may include directing a laser beam to solidify selected areas of the material by discontinuously and sequentially solidifying sub-millimeter sized sections of the areas such that the continuously solidified sections are spaced apart.

The method may include directing the laser beam to solidify the selected area of the material by advancing the laser beam along the scan path a plurality of times, wherein on each scan along the scan path the laser beam solidifies spaced apart segments of the scan path, each subsequent scan solidifying a segment between the segments solidified by the previous scan.

The method may include directing a plurality of laser beams to solidify the selected area of the material by successively advancing a plurality of the laser beams along a scan path, wherein on a scan of each of the laser beams along the scan path, the laser beams solidify spaced apart sections of the scan path, and a scan of one of the laser beams along the scan path solidifies sections between the sections of the scan path solidified by another of the laser beams.

The method may comprise directing a laser beam to cure selected areas of the material by irradiating sections of the areas with the laser beam such that each irradiated section is cured before adjacent sections are irradiated by the laser beam, wherein each section of the selected area of one layer is arranged to (only) partially overlap a section of the corresponding selected area of a preceding layer.

According to a third aspect of the invention, there is provided a data carrier having instructions stored thereon which, when executed by a processing unit of an additive manufacturing apparatus, cause the processing unit to control the additive manufacturing apparatus to perform the method of the second aspect of the invention.

The data carrier of the above aspect of the invention may be a suitable medium for providing instructions to a machine, for example a non-transitory data carrier, for example a floppy disk, a CD ROM, a DVD ROM/RAM (including-R/-RW and + R/+ RW), a HD DVD, a blu-ray (TM) disc, a memory (e.g. a memory stick (TM), an SD card, a compact flash card or similar), an optical disc drive (e.g. a hard disk drive), a magnetic tape, any magnetic/optical storage device or a transient data carrier, for example a signal over electrical or optical fibre or a wireless signal, for example a signal sent over a wired or wireless network (e.g. internet download, FTP transfer or similar).

Drawings

FIG. 1 is a schematic view of a selective laser solidification apparatus according to an embodiment of the present invention;

FIG. 2 is a schematic view of the selective laser solidification apparatus from the other side;

FIGS. 3a and 3b are schematic views illustrating scanning along a scan path;

FIG. 4 is a schematic diagram illustrating a fill scan of a region according to an embodiment of the invention; and

fig. 5 is a schematic diagram illustrating the fill scan of fig. 4 for multiple layers.

Detailed Description

Referring to fig. 1 and 2, a laser solidification apparatus according to an embodiment of the present invention includes a main chamber 101 having

partitions

115, 116 therein defining a build chamber 117 and a surface onto which powder can be deposited. A build platform 102 is provided for supporting an object 103 that is built by selective laser melting of powder 104. The platform 102 may be lowered within the build chamber 117 as successive layers of the object 103 are formed. The usable build volume is defined by the extent to which the build platform 102 can be lowered into the build chamber 117.

The layer of powder 104 is formed as the object 103 is built up by the dispensing apparatus 108 and the elongated scraper 109. For example, the dispensing apparatus 108 may be an apparatus as described in WO 2010/007396.

Laser module 105 generates laser light for melting powder 104, which is directed by optical scanner 106 under the control of computer 130, as needed. The laser enters the chamber 101 through a window 107.

The optical scanner 106 includes turning optics (in this embodiment, two

movable mirrors

106a, 106b) for directing the laser beam to a desired location on the powder bed 104 and focusing optics (in this embodiment, a pair of

movable lenses

106c, 106d) for adjusting the focal length of the laser beam. Motors (not shown) drive the movement of the mirror 106a and

lenses

106b, 106c, the motors being controlled by the processor 131.

Computer 130 includes a processor unit 131, a memory 132, a display 133, user input devices 134 such as a keyboard, touch screen, etc., data connections to modules of the laser melting unit such as optical module 106 and laser module 105, and an external data connection 135. Stored on the memory 132 is a computer program that issues instructions to the processing unit to perform the presently described method.

The processor receives, via external connections 135, geometric data describing the scan path taken in solidifying the powder areas in each powder layer. To build a part, the processor controls the scanner 106 to direct the laser beam according to the scan path defined in the geometry data.

Referring to fig. 3a and 3b, in this embodiment, to perform a scan along a scan path (e.g., boundary scan path 200) extending around an area of the material to be cured, the laser 105 is synchronized with the scanner 106 to expose a series of discrete points 201 along the scan path 200 to the laser beam. For each scan path 200, a dot distance d, a dot exposure time, and a dot size are defined. The direction D of the scanning spot 201 is also defined. In fig. 3a, the boundary scan path 200 is scanned twice in direction D, with spaced apart points 201a (shown in fig. 3a as filled in horizontal straight lines) exposed on a first scan of the laser beam along the scan path 200, and spaced apart points 201b (shown in fig. 3a as filled in dashed lines) between the points 201a exposed on a second scan of the laser beam along the scan path 200. In fig. 3a, the dots 201 (melt pools formed by the laser beams) are shown as non-overlapping for clarity, but in actual practice will overlap at least slightly so that lines of solidified material are formed along the scan path 200.

By solidifying each other spot 201 continuously along the scan path, the laser beam melts the material at the other spots 201 during the time that the melted material at each spot 201 is allowed to solidify before the material at the adjacent spot is solidified. Allowing the melt at each point 201 to solidify separately may allow for more precise lines of solidification to be formed. Specifically, the laser beam does not drag the melting front near the scan path 200, which can lead to inaccuracies and epitaxial or columnar grain growth.

In the embodiment shown in FIG. 3a, while each point 201 is being irradiated by a laser beam, the point of the laser beam remains substantially stationary at point 201, forming a substantially spherical molten pool. However, as shown in fig. 3b, forming spaced apart elongated molten pools less than 1mm in length by less movement of the spot of the laser beam across the powder bed before the laser beam closes and jumps to the next spaced apart section 210, 211 of the scan path 200 to be exposed may still achieve some accuracy advantages. To balance accuracy and performance, it may be desirable to form the elongated sections 210, 211 (rather than the discrete points 201).

It is believed that for typical laser parameters suitable for selective laser melting, the laser beam may radiate a section of less than 1mm, thereby forming a molten pool extending across the entire length of the section. In this way, the cured sections 210, 211 will have little directional properties. Beyond 1mm, the beginning of the segment will solidify before the end portion of the segment has been melted. The metallic material typically solidifies within 0.1ps to 1.66 ps. The speed of the laser beam depends on the energy that enables the laser beam to bind the material per unit time while avoiding excessive vaporization of the material. For a 500 watt laser focused to a 80 micron spot, the laser beam speed may be on the order of 2m/s to 500 m/s.

In fig. 3b, in a first scan along scan path 200, the laser beam irradiates spaced-apart segments 210, and in a second scan of the scan path, the laser beam irradiates spaced-apart segments 211 between segments 210.

In both fig. 3a and 3b, the first scan and the second scan are in the same direction. However, in an alternative embodiment, the first scan and the second scan are in opposite directions. Furthermore, in yet another embodiment, the dots or segments are spaced apart such that three or more scans must be made along the scan path to form a continuous curing line along the scan path.

Fig. 4 illustrates another scanning strategy for solidifying the core of region 303, in accordance with an embodiment of the present invention. A spot 301 is irradiated with a laser beam to solidify a region 303. The dots 301 are arranged in a 2-dimensional triangular pattern and the laser irradiates the dots in the order indicated by numbers 1 to 28 such that the successively irradiated dots 301 are spaced apart and the dots (or possibly multiple dots) between the successively irradiated dots are irradiated before the successively irradiated dots have time to cure, or the dots (or possibly multiple dots) between the successively irradiated dots are irradiated and cured before the successively irradiated dots are irradiated.

The dots 301 are scanned along a straight scan path (each column of dots 301) in one or two directions indicated by arrow D in the order shown in fig. 4. The spaced dots 301 are scanned in a first direction (page down) with a first scan path (left most column) and then in a second opposite direction (page up) with a second scan path (second column from right) with the spaced dots 301. The laser beam then returns to the first pass to continue this operation for all scan paths (columns of dots in direction D) until the entire region 303 is cured.

It will be appreciated that as with figure 3b, the core may be filled with separately irradiated elongate sections, rather than dots 301. Further, each scan along the scan path is not all in the same direction, and each scan along the scan path may be in the opposite direction.

Fig. 5 shows a fill pattern of three successive layers 402a to 402 c. The position of the dots 401 of each layer 402a to 402c is offset relative to the adjacent layers so that the centers of the dots 401 of adjacent layers do not coincide. As indicated by arrow D₁、D₂、D₃The straight scan path of the point 401 scanning each layer 402a to 402c is in the indicated direction, which is rotated between each layer 402a to 402 c. In this embodiment, the triangular pattern of dots 401 allows the direction to be rotated 60 degrees between each layer.

The scan order of the triangular pattern of

dots

301, 401 shown in fig. 4 and 5 may progress with the movement of the laser beam in orthogonal directions, rather than along a straight scan path in two opposite directions as described above. In this way, the successively irradiated

dots

301, 401 of the pattern are arranged at intervals in direction D and in a direction orthogonal to D.

Furthermore, the offset pattern shown in fig. 5 may provide benefits even in a scanning order in which adjacent dots are continuously irradiated.

It will be understood that variations and modifications may be made to the above-described embodiments without departing from the scope of the present invention as defined herein. For example, an additive manufacturing apparatus may include a plurality of laser beams and a scanning module for independently steering each laser beam. In the embodiment shown in fig. 3a and 3b, each scan along the boundary scan path may be through the same laser beam or a different laser beam. In particular, the second laser beam may begin scanning along the scan path before the first laser beam has completed scanning of the laser path, the two scans being sufficiently spaced apart that the segment irradiated by the first laser beam has been cured before the second laser beam begins curing an adjacent segment along the scan path. In a second embodiment, illustrated in fig. 4 and 5, multiple scanning strategies may be used with multiple laser beams. The laser beams may be scanned along the same path, or alternatively, a more complex scanning strategy may be used in which each laser beam is advanced along a different scan path (which may or may not partially overlap).

Claims

1. A selective laser melting additive manufacturing apparatus comprising:

a build chamber;

a build platform that is lowerable in the build chamber such that a layer of flowable powder material can be continuously formed across the build platform;

a laser for generating one or more laser beams;

a scanning unit for directing the one or more laser beams onto each layer to selectively consolidate the powder material; and

a processor for controlling the scanning unit, the processor being arranged to control the scanning unit to direct the one or more laser beams to melt selected areas of powdered material by irradiating sections of the selected areas with the one or more laser beams non-continuously and in an order such that successively solidified sections are spaced apart,

characterized in that each section is a substantially circular spot, the spots of each layer are arranged in a regular triangular pattern, each spot is sized such that a molten pool spans the entire spot, and each molten spot is allowed to solidify before irradiating the layer with one of the laser beams or other laser beams to melt adjacent and overlapping spots.

2. The selective laser melting additive manufacturing apparatus of claim 1, wherein the triangular pattern of one layer is offset from a corresponding triangular pattern of a previous layer.

3. A selective laser melting additive manufacturing apparatus according to claim 1 or claim 2, wherein the processor is arranged to control the scanning unit such that the dots of the regular triangular pattern are irradiated with the one or more laser beams advancing across the regular triangular pattern in a direction different from a direction in which the one or more laser beams advancing across the regular triangular pattern in the corresponding selected region of the previous layer.

4. The selective laser melting additive manufacturing apparatus of claim 3, wherein the direction of advancing across the triangular pattern to irradiate the dots changes by 60 degrees or 120 degrees between each layer.

5. The selective laser melting additive manufacturing apparatus of claim 1, wherein each segment is formed by irradiating a single point with the laser beam.

6. The selective laser melting additive manufacturing apparatus of claim 1, wherein each segment is formed by moving the laser beam across the layer.

7. A selective laser melting additive manufacturing apparatus according to claim 5 or claim 6, wherein each section of a selected area of one layer is arranged to partially overlap a section of a corresponding selected area of a preceding layer.

8. The selective laser melting additive manufacturing apparatus of claim 7, wherein each segment is a substantially circular dot, the dots of each layer being arranged in a regular pattern, wherein the pattern of one layer is offset relative to a corresponding pattern of the previous layer.

9. The selective laser melting additive manufacturing apparatus of claim 8, wherein the dots are arranged in a triangular pattern.

10. The selective laser melting additive manufacturing apparatus of claim 8 or claim 9, wherein the laser beam advances across the pattern in a direction to irradiate the dots of the pattern that is different from a direction in which the laser beam advances across the pattern in the corresponding selected area of the previous layer to irradiate dots.

11. A method of scanning layers of material in a layer-by-layer selective laser melting additive manufacturing process in which successive layers of flowable powdered material are formed across a build platform and one or more laser beams are scanned across selected regions of each layer to consolidate the powdered material in selected regions, the method comprising directing the one or more laser beams to melt selected regions of powdered material by irradiating segments of selected regions with the one or more laser beams non-continuously and in an order such that successively solidified segments are spaced apart,

12. The method of claim 11, wherein the triangular pattern of one layer is offset from a corresponding triangular pattern of a previous layer.

13. A method according to claim 11 or claim 12, wherein the dots of the regular triangular pattern are irradiated with the one or more laser beams advancing across the regular triangular pattern in a direction different from the direction in which the one or more laser beams advancing across the regular triangular pattern in the corresponding selected area of the previous layer.

14. The method of claim 13, wherein the direction of advancing across the triangular pattern to radiate the dots varies by 60 degrees or 120 degrees between each layer.

15. A selective laser melting additive manufacturing apparatus, the additive manufacturing apparatus comprising:

a build chamber;

a laser for generating a laser beam;

a scanning unit for directing the laser beam onto each layer to selectively consolidate the powder material; and

a processor for controlling the scanning unit, the processor arranged to control the scanning unit to advance the laser beam multiple times along a scan path to direct the laser beam to melt a selected region of powder material,

wherein on each scan along the scan path, the laser beam melts spaced apart sections of the scan path, each subsequent scan melts a section located between the sections melted by the previous scan, allowing each melted section to solidify before adjacent and overlapping sections are melted, wherein each section is sized such that a molten pool spans the entire section.

16. The selective laser melting additive manufacturing apparatus of claim 15, wherein the scan path is a boundary scan path around a boundary of the selected region.

17. The selective laser melting additive manufacturing apparatus of claim 15, wherein a first scan of the laser beam along the scan path is in a first direction and a subsequent scan of the laser beam or another laser beam along the scan path is in a second, opposite direction.

18. Selective laser melting additive manufacturing apparatus according to claim 15, wherein the processor is arranged to control the scanning unit to direct the laser beam/beams to melt the selected region of the powder material by melting sub-millimeter sized sections of the selected region non-continuously and sequentially such that successively melted sections are spaced apart.

19. The selective laser melting additive manufacturing apparatus of claim 15, wherein each segment is formed by irradiating a single point with the laser beam.

20. The selective laser melting additive manufacturing apparatus of claim 15, wherein each segment is formed by moving the laser beam across the layer.

21. The selective laser melting additive manufacturing apparatus of any one of claims 15 to 20, wherein each section of a selected area of one layer is arranged to partially overlap a section of a corresponding selected area of a previous layer.

22. The selective laser melting additive manufacturing apparatus of claim 21, wherein each segment is a substantially circular dot, the dots of each layer being arranged in a regular pattern, wherein the pattern of one layer is offset relative to a corresponding pattern of the previous layer.

23. The selective laser melting additive manufacturing apparatus of claim 22, wherein the dots are arranged in a triangular pattern.

24. The selective laser melting additive manufacturing apparatus of claim 22 or claim 23, wherein the laser beam advances across the pattern in a direction to irradiate the dots of the pattern that is different from a direction in which the laser beam advances across the pattern in the corresponding selected area of the previous layer to irradiate dots.

25. A selective laser melting additive manufacturing apparatus, the additive manufacturing apparatus comprising:

a build chamber;

a laser source for generating a plurality of laser beams;

a processor for controlling the scanning unit, the processor being arranged to control the scanning unit to advance a plurality of the laser beams along a scan path,

wherein on a scan of each of the laser beams along the scan path, the laser beams melt spaced-apart sections of the scan path, and a scan of one of the laser beams along the scan path melts a section located between sections of the scan path melted by another of the laser beams, allowing each melted section to solidify before adjacent and overlapping sections are melted, wherein each section is sized such that a melt pool spans the entire section.

26. The selective laser melting additive manufacturing apparatus of claim 25, wherein the scan path is a boundary scan path around a boundary of a selected region.

27. The selective laser melting additive manufacturing apparatus of claim 25, wherein a first scan of the laser beam along the scan path is in a first direction and a subsequent scan of the laser beam or another laser beam along the scan path is in a second, opposite direction.

28. The selective laser melting additive manufacturing apparatus of claim 25, wherein the processor is arranged to control the scanning unit to direct the laser beam/beams to melt selected regions of the powder material by melting sub-millimeter sized sections of the selected regions non-continuously and sequentially in order such that continuously melted sections are spaced apart.

29. The selective laser melting additive manufacturing apparatus of any one of claims 25 to 28, wherein each segment is formed by irradiating the laser beam at a single point.

30. The selective laser melting additive manufacturing apparatus of any one of claims 25 to 28, wherein each segment is formed by moving the laser beam across the layer.

31. A selective laser melting additive manufacturing apparatus according to any one of claims 25 to 28, wherein each section of a selected area of one layer is arranged to partially overlap a section of a corresponding selected area of a preceding layer.

32. The selective laser melting additive manufacturing apparatus of claim 31, wherein each segment is a substantially circular dot, the dots of each layer being arranged in a regular pattern, wherein the pattern of one layer is offset relative to a corresponding pattern of the previous layer.

33. The selective laser melting additive manufacturing apparatus of claim 32, wherein the dots are arranged in a triangular pattern.

34. The selective laser melting additive manufacturing apparatus of any one of claims 32 or 33, wherein the laser beam advances across the pattern in a direction to irradiate the dots of the pattern that is different from a direction in which the laser beam advances across the pattern in the corresponding selected area of the previous layer to irradiate dots.

35. A selective laser melting additive manufacturing apparatus, the additive manufacturing apparatus comprising:

a build chamber;

a laser for generating a laser beam;

a processor for controlling the scanning unit, the processor being arranged to control the scanning unit to direct the laser beam to melt selected areas of powdered material by non-continuously melting sections of the selected areas, the sections forming a filling pattern for solidifying the selected areas,

characterised in that each section is of sub-millimetre size such that the molten pool spans the entire section, and the sections are irradiated in sequence such that successively melted sections are spaced apart and each section is allowed to solidify before adjacent and overlapping sections are melted.

36. The selective laser melting additive manufacturing apparatus of claim 35, wherein each segment is formed by irradiating a single point with the laser beam.

37. The selective laser melting additive manufacturing apparatus of claim 35, wherein each segment is formed by moving the laser beam across the layer.

38. A selective laser melting additive manufacturing apparatus according to any one of claims 35 to 37, wherein each section of a selected area of one layer is arranged to partially overlap a section of a corresponding selected area of a preceding layer.

39. The selective laser melting additive manufacturing apparatus of claim 38, wherein each segment is a substantially circular dot, the dots of each layer being arranged in a regular pattern, wherein the pattern of one layer is offset relative to a corresponding pattern of the previous layer.

40. The selective laser melting additive manufacturing apparatus of claim 39, wherein the dots are arranged in a triangular pattern.

41. The selective laser melting additive manufacturing apparatus of any one of claims 39 to 40, wherein the laser beam advances across the pattern to irradiate the dots of the pattern in a direction different from a direction in which the laser beam advances across the pattern in the corresponding selected area of the previous layer to irradiate dots.

42. A method of scanning layers of material in a layer-by-layer selective laser melting additive manufacturing process in which successive layers of flowable powdered material are formed across a build platform and a laser beam is scanned across a selected region of each layer to consolidate the powdered material in the selected region, the method comprising directing the laser beam to melt a selected region of the powdered material by advancing the laser beam along a scan path a plurality of times, wherein on each scan along the scan path the laser beam melts spaced apart sections of the scan path, each subsequent scan for melting sections located between sections melted by the previous scan allowing each melted section to solidify before adjacent and overlapping sections are melted, wherein each section is sized such that a melt pool spans the entire section.

43. A method of scanning layers of material in a layer-by-layer selective laser melting additive manufacturing process in which successive layers of flowable powdered material are formed across a build platform and a plurality of laser beams are scanned across selected regions of each layer to consolidate the powdered material in the selected regions, the method comprising directing the laser beams to melt selected regions of the powdered material by advancing a plurality of the laser beams successively along a scan path, wherein, on a scan of each of the laser beams along the scan path, the laser beams melt spaced apart sections of the scan path and a scan of one of the laser beams along the scan path melts a section located between sections of the scan path melted by another of the laser beams, allowing each melted section to solidify before adjacent and overlapping sections are melted, wherein each section is sized such that the molten pool spans the entire section.

44. A method of scanning a layer of material in a layer-by-layer selective laser melting additive manufacturing process, wherein successive layers of flowable powder material are formed across a build platform and a laser beam is scanned across selected areas of each layer to consolidate the powder material in the selected areas, the method includes directing the laser beam to melt selected areas of the powder material by discontinuously melting sections of the selected areas, the sections forming a filling pattern for solidifying the selected areas, wherein each section is sub-millimeter in size such that the molten pool spans the entire section, and the segments are irradiated in sequence such that successively melted segments are spaced apart and each segment is allowed to solidify before adjacent and overlapping segments are melted by irradiating the layer with one of the laser beams or other laser beams.

45. A method according to any one of claims 42 to 44, wherein each section of a selected area of one layer is arranged to partially overlap a section of a corresponding selected area of a preceding layer.

46. A data carrier having instructions stored thereon that, when executed by a processing unit of an additive manufacturing apparatus, cause the processing unit to control the additive manufacturing apparatus to perform the method of any one of claims 11 to 14 and 42 to 45.